Thornycroft 55ft CMB torpedo @ 1:20 scale.

I’ve decided for my next project to tackle a boat very different to my usual builds.

The model(s) will be 100% scratch built direct from plans.

I will add some history as I prepare the build so keep watching for updates.

As prep work for my new project, I’ve made a start enlarging the bulkheads, easily done with the computer, however I will need to draw out manually the keel and other strengtherners.

This will be a slow build over the next few years, a lot of template and mock ups before commiting to material.

The boat has started to be scaled from the drawings 2.5x the size ( this is actully the max size I can swing a model in the workshop without tilting it about) making the model an overall size of 17.25cm wide, 11cm from keel to deck ( excluding bridge) and an overall length of 85.25cm.

A good size model for a hydroplane with a stepped hull, which should function and perform well on the water with little difficulty’s.

Some more progress started today, I’ve ensured the bulkhead plan is accurate to the rescale, so tonight I’m going to start cutting out the bulkheads and mount to some card, and re-cut again from the card templates I’ll be able to mark up keel and chine rail slots and working with the drawings should be able to make up the torpedo bay and wheelhouse should those fall on top of a bulkhead.

Again slow progress will continue with this model as I will be pretty much be building it from card to ensure the bulkheads and keel frames are accurate and correct, if they are only then I will commit to material.

But for now lots of copy’s of the bulkheads to cut out and remount...

I’ve been saving breakfast boxes and other cards for weeks so I have plenty of card of different available to pretty much mock up and entire frame made of card!

This should be interesting!

11 frames later.... notice frame 5 is produced twice once before the hull step the other behind, it actully took me a while to work out how that drop worked as the plan shows the step with a lean to stern, which won’t be an issue to install but I had to recalculate the bulkhead position to account fro the step, obviously this was a waste Of time as once I make up the keel frame template that will incorporate the actual step and positioning the smaller stepped frame is just stuck hard to the back of the forward frame...

I also think I need to tweet frame 4 on the top as it top profile is a minor curve compared to frame 5 which is very dramatic and I k ow planks do not twist that violently over that short of distance, it’s possible I overlooked a cut line on frame 4 which fortunately is an easy fix on the card rather than readjusting several frames to suit. I’ll keep working on that frame I’m sure it’s a minor adjustment just to decrease the twist somewhat over the hull length.

Honestly the line drawings once blown up to this scale do make the lines harder to distinguish however I have checked every forward bulkhead and none have been cut on that corner to that mark, so clearly that’s the corner I missed which makes the profile less aggressive and now correct. I’ll recut this frame and remount rather than try to bodge a symmetrical cut.

Once down I’ll cut these frame from their mounts and start to work on keel slot and chine stringer positions but as a markup only with no cuts yet, this is simply due to not knowing exactly which thickness of ply I require at present until I can resize the keel, my original thought was to use 5-6mm ply making it a heavy weight frame ( frames will be hollowed out during materials commitment) however the enlarged frames show a thick black line on the central with a width of 3mm almost... this I expect will be the ply thickness for frame work... again lots of unknowns but this comes with the territory of a scratch build.

I need to also point out that although the real boat is double planked, I may attempt to skin the boat with .5 or 1mm ply first and then reskin with planks, it’s not for a material saving reason it’s purly a time and strength method, the larger sections of base ply skin will make the boat more rigid and should drastically reduce plank movement over time... again this is also not really needed as I will be glass weaving the boats exterior however I do enjoy plank lines showing on a model such as this ( and it like that on the original boats) but the glass skin gives me that full water tightness and security, it will also make the boat a little easier for repairs should they be needed.

I must point out this boat I will not be keeping, its journey starts in my shop but ends in my bothers and fathers hands for long term display and usage. ( this version being a ww2 varation, a 2nd model will be made but of a singular torpedo drop and open cockpit in a ww1 variation for me to keep but this will be done later)

Frames re-cut from their card backings, and frame 4 was re-printed cut and mounted with the correct corner cut.

Today I’ll mark up the torpedo slot between frames 10-7 as the torpedo bay is imbedded Into the hull but yet not marked on the bulkhead plan, the cuts for these will also need calculation for material thickness to give me the correct cut for the final shape.

I also need to calculate the bulkhead from 9 upwards for spacing to fit rudder linkage and such, as these bulkheads are much smaller given the boats size taper to the rear along with the space lost to the torpedo bay cutout.

This will be a tight space long term for working in fortunaly it’s only the rudder assembly to be houses in this rear section,The bulkheads between 3-4-5-6 house the motors ( this boat being twin prop) and battery will lay either over top of the shafts between frames 5 and 8 or possibly behind frame 3 which is accessible via a deck hatch, or the later distance which can be the wheel house, again a lot of un decided things going on but planning forward helps eliminate issue later or at least reduce them.

For now I’ll focus of the rear and work my way up, once I’m happy with those positions and markings I’ll start the chine and keel slots, then enlarge the keel itself, which I hope to make the keel wrap from base of boat, the bow itself and whip over to be part of the central deck support, this may of course change and I’ll mark up a brest hook or even chine rail in but there are several hatches and openings along the forward deck which i’ld like to utilise at least for practicality of operation the boat.

They have had their designated slots and hollows cut, the larger frames have had their backs cut as they represent hatches, wheel house and torpedo bad insets, the rest of the frames simply hollowed leaving a fair chunk of material extra.

I’ll start making up the keel slot and chine rails later this evening and that I hope should be the frames templated. I may have to add extra chine supports along the inside torpedo bay for extra material support, and possibly around the hatch ways, I will attempt at lest around hatch’s to get chines to do the work for supports.

But for now this is the result of today’s efforts.

Post will at times be in rapid succession to keep your guys upstate along with keeping my memory fresh as there is a lot to workout and my brain can only remember so much!

I went to redraw the keel to its enlarged size and found all the bulkheads were oversize by around 4mm in height, and a few are a also 4mm to wide, Ive made a start to remedied every bulkheads central height and widest point width and marked out a 2mm permimiter around each frame, tomorrow I’ll re-cut the frames reduced width, fold them over and check each for mirror image of the marked side.

This error is due to the thickness of the enlarged drawing lines mostly so I can only assume that my cuts were that of the outter rather than the inner of the line.

Once I’ve done the frames recut and rechecked, should things still be off I’ll cut the bulkheads half that is correct redraw and remount to card again.

I’ld like to say this isn’t a big deal but sadly it’s a mistake that maybe fixed easily but this is the advantage of working with card... only time lost and working out how the enlarged plan works once blown up to this scale.

I’m pleased to say that after much cutting, and checking over and over the frames are now totally accurate in both height and width to the plan at its scaled version.

I may need to under cut the torpedo bay a little on the ply once i get to that stage so for now I’ve only taken a little material extra off the template, cutting this section slightly oversize will not matter as it’s and easy flat cut to pull it down later, after all it’s rasier to remove material than add it!

I’ll spend some more time today redrawing the keel and bow forward piece which should bow line up with bulkheads.

I also need to remember at this point to oversize the bow by 4-5mm to allow for the exposed keel, again it’s esier to go over size and pull it back than to add that later. The planking dose not overlap the bow, it’s a standard rebate so a thicking peice from thinner material will also need to be made to allow the planks to hook in, this I cannot slot out simply due to the lack of material thickness.

Overall I’m happy with how the frames are now, I’ll remount them to new card and get to the keel.

The first design of this boat was a single torpedo varation built at 45’ designed by thornycroft at the start of ww1 and the first launched in 1914, the last remaining version is currently in the imperial war museum under restoration.

During successful engagements this design was later converted into a twin torpedo carrier during ww2, the variations for wheel house design varied from open cockpit, to an enclosed wooden framework until the later designs carried a more “armoured design”

These early cmbs predate mtbs being the first of their kind, and subsequently thornycroft’s company designed what we know today as the thornycroft “Vosper” mtb.

These distinct cmbs came in a range of functions and only a few were built to carry torpedos to which again a unique design compared to more well known mtbs by storing the torpedos in a bay sunk into the deck, torpedos (ww2 variation carried 2x 18” torpedos) were launched backwards off the boat via a ram, which connected to a pin which armed and activated the prop motor, these cmbs were designed for stealth fast attack and the method for delivering the torpedo to the target was to line up the cmb to the target drop the torpedo and hard turn away leaving the torpedo to speed off in target. This again is not what we’re used to seeing today with torpedo laughed from deck tubes directly towards the target, but these cmbs were not of typical design like the vospers or more modern mtb’s these cmbs were designed as hydroplanes with a stepped hull and using twin experimental aircraft engines ( thornycroft Ry 12) could reach speeds of 42 knots.

Many of these boats were just that costal defence boats, however some warships were converted to hold one on its davits for more extended distance assaults ( more so during ww2)The cmbs with torpedos were basically a hit and run craft and with limited (2) torpedos in its later use made use on warships practical as a quicker reload could be achieved, however many launched from a warship never returned, a sad fact of war.The cmbs even the torpedo variations also had a deck machine guns ( .303 machine guns) although information about the guns use is said to have been useless due to the boats design on the sea and a clear shot could never be achieved.

In the latter stages of the cmbs life the vessels were closely used to shuttle agents for infiltration missions ( spy’s) and some were used to pick up stranded/ rescue men from enemy shores.Other methods these boats also endured due to their speed was to shuttle men and wounded during the d-day landings using the empty torpedo bay they could carry up to 10 men although some photos exist of it carrying less as a man on a stretcher with a pair of medics would take up a single bay slot.

A somewhat unique boat in its own right and it’s uses varied making this a successful predesessor to what we know today.It’s also strange for such a boat to not be so well known nor documented compared to others yet it’s function and ability were highly recognised & commended.

The only remaining version of the ww2 variation is 331 and iha been restored and currently resides at the duckford air museum in Kent.

7 of these torpedo boats were destoyed in action with all hands lost...

#67 lost 23/5/41, suda bay, bombed by aircraft#68 lost 14/12/41 via collision with mtb 215 off Libia #213 214 lost 23/5/41 sunk by German aircraft at suda bay#215 lost 29/3/42 for unknown reasons in the Mediterranean.#217 lost 23/5/41 sunk by German aircraft at suda bayAnd finally #216 ( to which these are the plans I’m working from are based (it also covers hull variations 67, 68 & 213-217, and hull 331 is the same however that contained a open cockpit with a steel bullet shield but is not marked in this plan) had the same fate as 217 during a joint mission.

WW1 single variation

WW2 cmb 331 ( renamed to cmb 4)

Notice the hull similarity’s but wheel house design varies greatly with the boat on the right supporting an “armoured surround” over the right supporting the hard wood surround

Armoured surround variation again being launched from dry dock & ww1 single toro variation used as a troop carrier.

After a bit of hectic day with baby scans, I finally managed to get to the shop this evening and have drawn the boats profile in full scale.

This in turn made it simple to produce a keel pattern for the boat, slots marked for every bulkhead.

Going by scale, the bulkhead frames should be that of 3mm ply ( it’s actully 2.8mm in true scale but 3mm is closest in stock sheet available) and the keel at the stem should be 6.25mm thick ( due to the cut in for the planking) but the keel itself is only again 3mm, this however I will reinforce with 3mm ply doublets each side between bulkheads, and Just have a 3mm stem exposed and apply some 2mm ply each side once planking is finished to complete the bow stem, it will be a tad oversize but I can live with that deviation to accurate scale for the sake of the correct look rather than true to scale, this is simply due to planking and skinning the boat to keep it exactly to scale I would need to skin the boat to a maximum of 1.5mm thickness, although possible that regardless is a very flimsy hull especially for a boat this long and with this profile, so I have opted to skin the boat with 0.5mm ply and then plank with 2.5mm strips giving a 3mm thick skin overall, and once a layer of glass weave added should be a strong hull.

The overall discrepancy is +/-1mm and no one will notice the boat being 1mm thicker than it’s supposed to be!

All that’s left now is to mark up the bulkhead slots now I know the keel slots and then start work on the chine placement, then back this keel with two layers of card for a template and I should be ready to attempt a mock-up before commiting to material...

This I have to say although it’s been a lot of hours work I have none the less enjoyed scaling this boat and turning the official boat drawings into a model... rewarding!

I do hope you guys are enjoying this, please feel free to ask questions or give input, I’ve scratch built before but not had the chance to have to build a full frame kit from scratch.

I’ve also taken the time to mix some wood glue and black paint mix and seal the template edges, this will help stop the card backings from delaminating somewhat over use and also stiffens up the edges so they can be used more than once accurately without the card starting to take abuse, this is more for when I get around to doing the 2nd boat which will be after I’ve done the first.. this helps me also mod the template as I go for the 2nd for any issues I may encounter later.

The keel is 33” long I will try to source ply big enough for a singular cut but that comes with added cost, I may due to the fact the keel will have reinforced strips from bulkhead to bulkhead slice the keel into possibly 2 or 3 sections, as I did buy a stack of 3,4 & 6 mm birch ply in a4 and 1x2’ on eBay for a reasonable price ( it worked out cheaper than a couple of larger 4x4’ sheets, which also wouldn’t have been a good quality birch)

It a lot done today in regards to progress, I’ve had very little sleep with my eldest deciding to throw up all night, but I did take an hour this afternoon to cut some thinner slots in the bulkheads that slide to the keel slots, the bulkhead slots are thinner simply due to mock-up I’ll expand them to the correct size once transferred to ply.

As a test I’ve mocked up the frames do the keel so I can decide on the best course of action in regards to a build board and also to view if the use of a Brest hook is needed in addition to Chines and stringers.

But the mocked up frame even in its loose form has an impressive shape and stance, and I’m looking forward to moving into material and planking this machine.

Haven’t done much in a couple of days to any builds the heat has made it somewhat unbearable to do any real work..

But I did take half hour today to test one of the bulkhead frames on some scrap ply I had hanging about..

I took the first bulkhead (smallest) traced and cut it out on the fret saw...

Overall the bulkhead came out pretty good and I have the edges a light sand, I did notice tho this bulkheads left hand side the top curve is not exact to the other side it’s lowest central point is a tad higher than the right..

Although it’s slightly off it’s alao not the end of the world, I’ll take this test and mark up notes, so I can trace out the bulkheads, flip over and ensure they are all mirror images when I do final tracing, and cutting out will be done slightly oversize and I’ll sand back to the finished size.

This should ensure both sides are the same but the template gives me the basic shape, with some final shaping done to each I should have perfect bulkheads.

This I must note is the first time I’ve used my fretsaw properly for quite some time and I’m a little out of practise. I’ll also note the blade currently fitted has been there for almost 2 years so it’s a little worn and a little twisted in places, so I’ll be sure to swap that out to a brand new blade before commiting to the final build.

Overall I’m pleased with my test piece and this shows the templates will do a reasonably accurate job.

Bit of trial and error... I will try to source some other scrap ply and trial each bulkhead if possible to see how many need some tweaking if I can be bothered to go that far... that test maybe more for sake of frame 5 and 5 1/2 as they are step frames and need to align perfectly at the top butbthe base are totally different shapes due to the step, I know my template on the top is a little off on the cutout but I took that into account as i could adjust it depending on where I inserted a chine or stringer rail for the hatch.. it’s also not a massive deal with those bulkheads being slights off in regards to the opening as the hatch that sits over that section is actully wider than the hole so I have plenty of play either side for s tweek.

Again the joys of creating things totally from scratch and trying to preplan!